This invention is related to an optically addressed spatial light modulator and a method for changing the relative sensitivity of an optically addressed spatial light modulator using a ferroelectric liquid crystal material and so on as a light modulation material in the fields of optical information processing and optical measurement.
In the fields of optical information processing and optical measurement, as researches have progressed, there arises a need for a spatial light modulator having high resolution and fast response time. Conventionally, an electro-optic crystal material such as a BSO crystal material (Bi.sub.12 SiO.sub.20 crystal) has been used as a light modulation material of many optical addressing spatial light modulators, and a liquid crystal light valve using a nematic liquid crystal material has also been employed as an optical addressing spatial light modulator in many cases. However, these materials are not sufficient to meet the above requirements relating to resolution and response time. Then, recently there has been developed and already used an optical addressing spatial light modulator using a ferroelectric liquid crystal material as the light modulation material (hereinafter abbreviated as "FLC-OASLM").
First of all, the structure of the FLC-OASLM will be explained. The FLC-OASLM is different from the conventional liquid crystal light valve using the nematic liquid crystal material in respect of using, as a liquid crystal layer, the ferroelectric liquid crystal material which has clear bistability between light transmittance or light reflectance and an applied voltage. FIG. 2 is a sectional view showing the structure of the FLC-OASLM. On the surfaces of transparent substrates 101a, 101b made of glass, plastic and so on for sandwiching liquid crystal molecules are provided transparent electrode layers 102a, 102b, and alignment layers 103a, 103b which are formed by evaporating silicon monoxide obliquely at an angle ranging from 75.degree. to 85.degree. with a normal direction of the respective transparent substrates. The transparent substrates 101a, 101b make the respective alignment layers 103a, 103b to be opposed by controlling a gap through a spacer 109 and to sandwich a ferroelectric liquid crystal layer. There are laminated a photoconductive layer 105, a light blocking layer 106, and dielectric mirror 107 on the transparent electrode 102a of an optical write side of the FLC-OASLM and under the alignment layer 103a. Anti-reflection coating layers 108a, 108b are formed respectively on the outsides of the transparent substrate 101a on the write side and the transparent substrate 101b on the read side, which constitute a cell.
Next, two types of methods for initializing the FLC-OASLM having the above structure are described. In the first method, an entire plane of the write side of the FLC-OASLM is once irradiated with light. A pulse voltage, a direct current bias voltage, or a direct current bias voltage which is superimposed with an alternating current voltage between 100 Hz and 50k Hz are applied as an erasing voltage between the transparent electrode layers 102a and 102b. These voltages are sufficiently higher than a threshold voltage at the time of irradiation. Then, all of ferroelectric liquid crystal molecules are arranged in one direction resulting in a stable status, and the status is recorded. In the second method, the FLC-OASLM is not irradiated with light at all. A pulse voltage, a direct current bias voltage, or a direct current bias voltage which is superimposed with an alternating current voltage between 100 Hz and 50k Hz are applied as an erasing voltage between the transparent electrodes 102a and 102b. These voltages are sufficiently higher than a threshold voltage at the time of no irradiation. Then, all of ferroelectric liquid crystal molecules are arranged in one direction resulting in a stable status, and the status is recorded. Generally, the threshold voltage at the time of no irradiation is higher than that at the time of irradiation.
Further, explanation will be given as to operations to be done after the FLC-OASLM is initialized as explained above. A pulse voltage, a direct current bias voltage, or a direct current bias voltage which is superimposed with an alternating current voltage between 100 Hz and 50k Hz are applied as a write voltage between the transparent electrode layers 102a and 102b. Those voltages have a polarity reverse to that of the voltage used for initialization, and are lower than the threshold voltage when light is irradiated, and are higher than the threshold voltage when no light is irradiated. While the write voltage is applied, and image is optically written in by laser beam and so on. Carriers are generated in the photoconductive layer 105 in the region irradiated with laser, and the carriers move toward an electric field. As a result, the threshold voltage declines, and an applied voltage which is higher than the threshold voltage and has a polarity reverse to that of the voltage used for initialization is applied to the region irradiated with laser. Then, the molecules reverse in the ferroelectric liquid crystal material accompanying the reverse of spontaneous polarization, and the ferroelectric liquid crystal material turns out from one stable status to another. Therefore, an image is binarized and recorded. The recorded image remains recorded even when a drive voltage becomes zero.
The image binarized and recorded in the above manner can be read out either in a positive or a negative by irradiating read light of linearly polarized light which is arranged so that its polarization axis should be in the direction of the liquid crystal molecules arranged in one direction by initialization (or in a direction perpendicular to the above direction), or by passing the reflected light of a dielectric mirror 107 through an analyzer which is arranged so that its polarization axis should be perpendicular to (or parallel to) the polarization direction of the reflected light. A polarization beam splitter is often used as an analyzer.
In theory, it is possible to initialize the FLC-OASLM and memorize an image in the above-mentioned method. However, as a practical method for driving the FLC-OASLM, such driving voltage as indicated in FIG. 3 is applied to the FLC-OASLM in order to record, erase and read out an image in many cases. FIG. 3 shows one example of driving voltage wave forms which are applied to the FLC-OASLM when the transparent electrode layer 102a on the read side is grounded. In the conventional FLC-OASLM, write light and read light always irradiate respectively the write side (the side of the transparent substrate 101a) and the read side (the side of the transparent substrate 101b) of the FLC-OASLM. The FLC-OASLM is initialized by being applied with a positive pulse voltage (referred to as an erasing pulse 31) which is an erasing voltage. A picture image is recorded by a negative pulse voltage (referred to as a write pulse 32) as a write voltage, and an image memorized by the write pulse 32 and zero voltage 33 is read out. With this method, the FLC-OASLM can be driven in the frequency range from scores of Hz to several kHz.
Actually, however, it is still difficult to make a FLC-OASLM which includes the dielectric mirror 107 or the light blocking layer 106 as a light reflecting and separating layer and is uniform in a large area, and the FLC-OASLM which does not include the light reflecting and separating layer is often used. One of the courses of this fact is that installing the light blocking layer 106 or the dielectric mirror 107 makes it difficult to control a gap of about 1 to 2 .mu.m for the injection of ferroelectric liquid crystal material and to control the alignment of the ferroelectric liquid crystal material. In the FLC-OASLM described above, a read out light is reflected on an interface of the photoconductive layer 105. The reflectance of the read-out light is approximately 20% if hydrogenated amorphous silicon is used as the photoconductive layer 105 and the wavelength of the read-out light is 633 nm.
As the photoconductive layer 105, single crystal BSO (Bi.sub.12 SiO.sub.20) or single crystal silicon is sometimes used, but hydrogenated amorphous silicon is used in many cases at the present stage. The reason for using it is that the response time as a spatial light modulator can be shortened, that resolution can be improved as the thickness of the photoconductive layer 105 can be thinned, and that the manufacture is facilitated.
However, a light addressed spatial modulator, especially the FLC-OASLM described above, has some problems as follows. When hydrogenated amorphous silicon is used as the photoconductive layer 105, for instance, the energy necessary for the record into the FLC-OASLM per minimum spot area is in the range from 0.03 pJ to 0.2 pJ and does not have a multiplication action as a photomultiplier does. It cannot be said that the recording sensitivity is enough in this case. In other words, the major factor in restricting the application of the FLC-OASLM is that an image cannot be recorded without the write light of fairly strong intensity.
When the FLC-OASLM is used as an incoherent to coherent transducer, as one example of the application, outdoor scenery, parts flowing along production lines in a factory and so on are imaged on the photoconductive layer 105 by the use of an imaging lens in order to record images of the scenery and the parts. However, the intensity of such a write light (intensity of the image imaged on the FLC-OASLM) is generally much too weak to record. Further, if a Fourier transformed image is recorded on the FLC-OASLM used in an optical pattern recognition system, a high frequency component of the Fourier transformed image has a remarkably weaker intensity compared with its low frequency component. Consequently, the high frequency component cannot be recorded, and accurate pattern recognition is difficult.
In the above case, images could be recorded by improving the sensitivity of the FLC-OASLM itself or by intensifying the intensity per unit area of the write light. Then, the write light intensity per unit area could be intensified by raising luminance of the light source illuminating an object or by reducing the size of a written image. However, it is very difficult to improve substantially the sensitivity of the FLC-OASLM itself and the luminance of the light source. Further, the size of the written image seldom can be substantially reduced because of problems related to the resolution and application of the FLC-OASLM. As explained above, in the conventional method the application of the FLC-OASLM cannot be expanded, for example, by application to the incoherent to coherent transducer, because it is difficult to intensify the intensity of the written image for the purpose of writing an image with weak write light intensity to the FLC-OASLM.
There is another problem in that the sensitivity is different depending on the wavelength of the write light, which is one of the spectral sensitivity characteristics. For instance, if hydrogenated amorphous silicon is used as the photoconductive layers 105, sensitivity is high on wavelengths approximately between 600 nm and 650 nm, but quite low on other wavelengths. As a result, when white light is used as the light source of the write light, an image cannot be recorded either, if the entire light intensity is strong but the spectral intensity of the wavelength on which sensitivity is high for hydrogenated amorphous silicon is weak. Moreover, there could be a case in which a wavelength with low sensitivity must be used as the write light for some applications. In such a case, recording of an image is difficult as well.
Further, it is known that the accuracy of pattern recognition greatly changes if the threshold for recording the written image is changed, when an image including noise or a Fourier transformed image is recorded on the FLC-OASLM which is used in an optical pattern recognition system. It is easy to increase the threshold because it is equivalent to reducing the write light intensity with an ND filter and so on. However, there is a drawback in that it is very difficult, on the contrary, to record the written image by lowering the threshold because the sensitivity of the FLC-OASLM is insufficient.
The above drawbacks are not limited to the FLC-OASLM, but similar drawbacks exist as to general optically addressed spatial light modulators.